1) Field of the Disclosure
The disclosure relates to a method and system for enhanced remote detection of low concentration vapors, and more particularly to a method and system for enhanced remote detection of low concentration vapors using a heating source and a sensor.
2) Description of Related Art
There is an ongoing need for methods and systems capable of early remote detection of hazardous and controlled solid and liquid materials, such as chemical agents, biological agents, explosive agents, narcotics and other hazardous and controlled substances, and in particular, solid and liquid materials having very low vapor pressures. Every solid and liquid material has some vapor pressure, with solid materials typically having very low vapor pressures at ambient or room temperature, and liquid materials having higher vapor pressures at ambient or room temperature. In particular, explosive solid materials such as TNT (trinitrotoluene), have a very low vapor concentration at ambient or room temperature. Such low vapor concentrations can make detection, and in particular vapor detection, of such materials difficult. The detection of such hazardous solid and liquid materials is important for security screening measures, forensic analysis, and environmental applications. The choice of the detection method or system used depends on the chemical agent, biological agent, or explosive agent being sought, any expected background interferences, and whether point samples or standoff distances are required. Local, non-remote detection methods and systems using point samples for detecting hazardous solid and liquid vapor materials exist that use low power infrared, ultraviolet, or visible light sources to detect and identify vapor materials based on their unique optical signatures, such as emission, absorption, and scattering. These methods and systems are capable of detecting solid and liquid material vapors at trace levels at parts per billion concentration levels or lower. However, active (laser-based) and passive (solar or thermal based) remote vapor detection and identification of extremely low vapor pressure, such as parts per billion concentrations and lower, solid and liquid materials under ambient or near room temperature conditions is currently not feasible due to the small interaction (e.g. light absorption) between the vapor material and the sensor radiation. In the case of an active remote sensing system, an increase in vapor detection probability can be achieved by increasing the laser power at a fixed vapor concentration. However, for a photon shot noise limited laser sensor, such as one whose light signal is large enough that the sensor system noise depends primarily on the statistics of the photons entering the sensor receiver aperture and not on detector system electronic performance in the sensor, the signal-to-noise ratio (SNR) increases only as the square root of the laser power, so that doubling the laser power of such a sensor increases the SNR only by the square root of two, or about a factor of 1.4. This is a modest gain compared to the order of magnitude or more improvement required to remotely detect extremely low concentration vapors. SNR is a primary determinant of the minimum detectable vapor concentration and of the probability of vapor detection for a laser sensor. Continuing to increase laser power while maintaining other essential capabilities and properties of the laser sensor system, such as narrow spectral line width, wavelength tunability, compact size, or power efficiency, is a major challenge. Another known method uses a large receiver telescope aperture to collect more of the light reflected, scattered, or emitted from the target when implementing active and passive remote sensing techniques, thus increasing the SNR. However, such large receiver apertures limit the portability of remote sensors, thereby limiting the platforms on which they can be deployed, and such large apertures may also decrease the ability to be covert in certain applications. In addition, as for the laser power, the sensor signal increases only as the square root of the receiver aperture area. Another known method uses lower noise photodetection hardware, such as lower noise photodiodes or cryogenically cooled detectors. However, while lower noise photodetection systems may increase the sensitivity of remote sensors, ultimate sensitivity is determined by photon shot noise, and a system that is independent of the sensor itself may be needed to overcome this limitation to further increase the probability of detection. Another known method uses integration of the signal over long time periods and/or averaging of multiple signal samples. However, such signal integration and sample averaging over long periods may have the limitation that the target or intervening atmosphere may be dynamically changing on a time scale that is short compared to the integration or sampling time, thus degrading the time resolution of the measurement. Moreover, sample-to-sample signal correlations set a limit to the achievable enhancement using this system. Another known method uses reduction of the distance between the sensor and the target material by either moving the sensor closer to the target material or moving the target material closer to the sensor. However, significantly reducing sensor-to-target distance is often not possible, such as for airborne applications and in the detection of hazardous and non-cooperative targets. In those cases where decreasing the range is possible, the SNR increases only linearly with decreasing range under photon shot noise limited conditions, which may not provide enough signal enhancement for many applications requiring ultra high sensitivity.
In addition, vapor detection methods and systems are known that use lasers to detect the vapor signatures of hazardous solid and liquid materials. However, the performance of known vapor detection sensors are often limited by the low vapor concentration of many solid and liquid materials under ambient conditions and by the often long standoff distances between the sensor and the target material.
Existing methods and systems do not sufficiently enhance the SNR of the measurement to render the target solid and liquid materials detectable at all ranges of interest using existing remote sensor technology, and in particular, to render target solid and liquid materials having very low vapor pressures at ambient temperature detectable. Known methods and systems may typically only increase the SNR by a factor of between 1 and 2, rather than by a factor of 10 or more, which is typically what is needed to detect very low vapor pressure materials. Thus, the increase in sensor SNR for known methods and systems is linear or sub-linear with changes in remote sensor parameters only, such as radiation power or receiver aperture area. Moreover, existing methods and systems only seek to improve or change the sensor itself rather than change the conditions of the material target for the purpose of improving detection probability.
Accordingly, there is a need for a method and system for enhanced remote detection of low concentration vapors that provides advantages over known methods and systems.